1. Field of the Invention
The present invention relates to an x-ray topography apparatus, which is an apparatus for displaying an x-ray diffraction image representing a two-dimensional image of a crystalline structure in the interior of an object.
2. Description of the Related Art
The two-dimensional, namely planar, x-ray diffraction image obtained using an x-ray topography apparatus is called an x-ray topograph. The morphological features revealed in the x-ray topograph typically represent structural features of the object. For example, lattice defects or distortion in single crystals appear as changes in x-ray intensity in the x-ray topograph. For this reason, x-ray topography apparatuses currently enjoy widespread use as apparatuses for carrying out “evaluation on integrity of crystals” in relation to single crystal materials.
For example, in the case of production facilities for Si (silicon) crystal, which is a single crystal material, the wafer diameter has progressively increased over time, and the larger diameters have resulted in an associated increase in size of the measuring instruments, i.e., x-ray topography apparatuses. Specifically, whereas Si crystals have a diameter of about 4 inches (101.6 mm) at the beginning, larger sizes of up to about 300 mm have recently been achieved. Even larger Si crystal sizes, e.g., up to about 450 mm, are anticipated in the near future.
Known x-ray topography apparatus include those disclosed, for example, in Japanese Laid-Open Patent Applications H11-014564 and 2000-314708. Japanese Laid-Open Patent Application H11-014564 discloses an x-ray topography apparatus adapted to produce an x-ray topograph on a flat x-ray fluorescent plate or a flat storage phosphor. Japanese Laid-Open Patent Application 2000-314708 discloses an x-ray topography apparatus adapted to produce an x-ray topograph on a flat x-ray film or flat CCD (charge coupled device) sensor.
There are a number of different types of x-ray topography apparatus. For example, known x-ray topography techniques include the Berg-Barrett method, which is a reflectional and single crystal technique; the Lang method, which is a transmissive and single crystal technique; and double crystal methods that use a crystal other than the sample as a monochromator or collimator. As regards the Lang method, a construction like that depicted in FIG. 9A, for example, is known.
The x-ray topography apparatus depicted in FIG. 9A, which is based on the Lang method, has an x-ray source 102 that emits x-rays to be directed onto a sample crystal 101, and also has an entry slit 103 positioned on the incident side of the sample crystal 101. X-rays are regulated by the entry slit 103 and enter the sample crystal 101. The light receiving side of the sample crystal 101 is provided with a diffraction slit 104 and a flat x-ray detection film 106. Previously, when the diameter of the sample 101 was smaller, the x-ray detection film 106 was a nuclear plate composed of a glass substrate coated with a thick coating (50-100 μm) of an emulsion, for example.
The entry slit 103 and the diffraction slit 104 are immovably fixed. As indicated by the arrow A, the sample crystal 101 and the x-ray detection film 106, in unison with one another, undergo scanning movement parallel to the sample face, i.e., the measured face, of the sample 101. During this scanning movement, as depicted by the fragmentary enlarged view in FIG. 9A, the x-ray detection film 106 is exposed by a diffracted x-ray R1 which has been diffracted at the lattice plane 105 of the sample crystal 101.
In this previous x-ray topography apparatus, the positional relationships of the sample crystal 101, the diffraction slit 104, and the x-ray detection film 106 are set such that an x-ray diffracted by the sample crystal 101 impinges in the vertical direction onto the x-ray detection film 106. The reason for doing so is that if an x-ray impinges obliquely on the thick emulsion coating of the x-ray detection film 106, the diffraction image may become blurred, and resolution may be reduced; therefore, the aim is to prevent such a decrease in resolution.
In order to make a diffracted x-ray impinge vertically on the x-ray detection film 106, it is thus necessary to position the diffraction slit 104 obliquely with respect to the sample crystal 101. On the other hand, the slit piece of the immobilized diffraction slit 104 must have a width that matches the size of the x-ray detection film, in order to prevent fogging by scattered x-rays. By doing this, during scanning of the sample crystal 101 and the x-ray detection film 106, there is risk of the sample crystal 101 colliding with the diffraction slit 104; however, when the diameter of the sample crystal 101 is small, the gap between the sample crystal 101 and the x-ray detection film 106 does not need to be very big, so this has not been a problem.
However, with a larger diameter of the sample crystal 101, the sample crystal 101 and the x-ray detection film 106 must be separated by a large gap in order to prevent collision of the sample crystal 101 and the diffraction slit 104. In such instances, a problem can be presented in regard to an excessively large gap being created at the end where a larger gap exists between the sample crystal 101 and the x-ray detection film 106, resulting in degraded x-ray optical resolution in this part.
To address this problem when a large-diameter sample crystal 101 is used, there have been adopted configurations such as that depicted in FIG. 9B, in which both the diffraction slit 104 and the x-ray detection film 106 are oriented parallel to the sample crystal 101. Such systems are widely employed in apparatuses for sample crystals 101 with diameters of 100 mm and above.
In this apparatus, while collision of the sample crystal 101 with the diffraction slit 104 may be avoided, the diffracted x-ray R1 obliquely impinges on the x-ray detection film 106, and diffraction image blur becomes a concern. However, by using an x-ray detection film 106 with a thinner emulsion coating, improved resolution is possible due to the smaller gap between the sample crystal 101 and the x-ray detection film 106, which more than compensates for degradation of resolution resulting from oblique incidence.
For example, in the case of MoKα rays impinging in a diagonal direction of 15° on an emulsion 10 μm thick while detecting reflection of Si 400, the degradation of resolution (i.e., blur) is on the order of 2.7 μm. Because the gap between the sample crystal 101 and the x-ray detection film 106 is kept to about 20 mm, the x-ray optical resolution is 10 μm. In contrast to this value, the value of 2.7 μm, which is effective for diagonal incidence, lies in the permissible range, and thus the resolution is adequate for practical purposes.
Where an imaging plate, which is a storage phosphor, is used instead of the x-ray detection film 106 as the x-ray detection element, the fact that the thickness of the sensing body corresponding to the thickness of the emulsion is 100 μm means that degradation of resolution due to diagonal incidence is on the order of 27 μm. The sensing resolution of the imaging plate is 50 μm, so the smaller image unsharpness of 27 μm is not considered a problem in practical terms. However, theoretically, vertical incidence of the diffracted x-ray on the imaging plate is ideal for obtaining high resolution topographs.
In instances where a flat x-ray sensing film is used, in order to protect the x-ray film or imaging plate from the phenomenon of fogging due to scattered x-rays, it is necessary to make the width of the diffraction slit piece larger. However, in this case the diffraction slit piece becomes larger and heavier, and it is necessary for the component supporting it to be robust, which has at times led to the problem that the x-ray detection section inclusive of the x-ray detection film and the like is excessively large and heavy.
In the field of electrostatic transfer apparatuses, there are known apparatuses adapted to produce an electrostatic latent image on the surface of a rotating photosensor drum (for example, Japanese Laid-Open Patent Applications H05-289190 and H10-313383). According to this invention, a planar x-ray detector of greater surface area than the sample is deformed into a cylindrical shape, but this component is completely different from the photosensor drums used in electrostatic transfer apparatuses.
From the above it will be appreciated that conventional x-ray topography apparatuses have problems such as the following.
(1) When a flat and planar x-ray detector is used in an arrangement in which the diffracted x-ray is incident from a perpendicular direction (e.g., as in FIG. 9A), resolution of the x-ray topograph is lower in the section where the gap between the sample and the x-ray detector is wide. If the sample has a large diameter, the gap between the sample and the x-ray detector must be expanded further in order to avoid collision with the diffraction slit during scanning, thereby further exacerbating the drop in resolution.
(2) When a flat and planar x-ray detector is used in an arrangement in which the diffracted x-ray is incident from an oblique direction (e.g., as in FIG. 9B), resolution of the x-ray topograph is reduced by the obliquely incident x-rays.
(3) In the apparatus depicted in either FIG. 9A or FIG. 9B, when the sample is of a large diameter, the surface area of the planar x-ray detector must be larger as well, making the x-ray detection section larger and harder to manipulate.